Synthesis, Characterization And Electroluminescent Properties Of Solution-processable Green Light-emitting Molecular Materials

Solution-processable molecular semiconductors have received increasing attention. Withrespect to their polymer counterparts, small molecular active compounds present theadvantages of a monodisperse unequivocal chemical structure and thus of a potentially betterreproducibility of synthesis and purification and more straightforward analysis ofstructurep?roperties relation-ships. Furthermore, when adequately solubilized by design thesesolution-processable molecular materials afford the possibility of low-cost large areaelectronics with respect to vacuum deposition-based device processing techlelogy.The research of this thesis has focused on the design, preparation and characterization ofnew efficient non-doped solution-processible green light-emitting molecular materials. Theemphases of molecular designs have been placed on the issues such as solution solubility,film-forming property, morphological stability, electron injection and transport,photoluminescent and electroluminescent efficiency and emission color purity.For the molecule design of electroluminescent materials, non-symmetrical moleculestructure may prevent the formation of molecular aggregation state thus increasingluminescent efficiency. We first synthesized a series of green light-emitting compoundswhich were named as 1a and 1b, based on an asymmetrically 4,7-disubstituted2,1,3-benzothiadiazole. The building block containing the derivative of carbazole wasintroduced on the 4-position of the core 2,1,3-benzothiadiazole core, while the alkoxysubstituted phenyl dendritic block was introduced on the 7-position of 2,1,3-benzothiadiazole.The devices with structure of ITO/PEDOT: PSS (50 nm)/PVK (40 nm)/1a (45 nm)/Ba (4nm)/Al (120 nm) exhibited a current efficiency of 10.6 cd/A. The color coordinates of thehigh-performance green emission were (0.34, 0.58) which were very close to pure greenemission.Based on the above results, it was noticed that the efficiencies of compounds 1a and 1bdecreased significantly in in double-layer devices with PVK at high current density. In orderto understand the properties of charge transport, we synthesised compounds 1c and 1dthrough introducing the derivative of carbazole and alkylfluorene at both the 4-position and7-positin of the core 2,1,3-benzothiadiazole. In comparison with compounds 1a and 1b inChapter 2, we prepared the similar device characterizations in the device structure:ITO/PEDOT: PSS (50 nm)/1a (1b, 1c, 45 nm)/TPBI (AlQ3, 30 nm)/LiF (2 nm) /Al (120 nm).We improved the performance of the devices. Compound 1a showed a high current efficiencyof 12.8 cd/A while the electron transport layer TPBI was introduced in the electroluminescent devices. The efficiency roll-off was improved and the efficiency decreased more slowly athigh current density. The current efficiency still maintained on a level of 11.2 cd/A at acurrent density of 20 mA/cm2. The maximum brightness of all devices reached 29271 cd/cm2.To simplify the synthesis, we synthesised two small molecular emitters CzFBTB andNBBTB with the derivative of carbazole and 1,3-di(naphthalen-1-yl)benzene which bear asimple structure and trend to form amorphous film. The two molecules with 2,1,3 -benzothiadiazole core exhibited intrinsicly amorphous glass state owing to the rigidmolecular dendrons at one end of molecule. The solubility of compounds was ensured bysolubilized diphenylphenyl moiety at the other end of molecule. The electroluminescentdevice ITO/PEDOT: PSS (50 nm)/PVK (40 nm)/CzFBTB (45 nm)/Ba (4 nm)/Al (120 nm)showed a current efficiency of 3.5 cd/A.A new asymmetrically substituted green emitter NCFPBT was prepared by theintroduction of the core 4,7-diphenyl-2,1,3-benzothiadiazole, which preserved hole injectionability and the film-forming property of material. The compounds obtained a relatively bettercolor purity and efficiency in electroluminescent devices. In the device structure: ITO/PEDOT: PSS (50 nm)/PVK (40 nm)/NCFPBT (45 nm)/CsF (2 nm)/Al (120 nm), thecompound NCFPBT exhibited a maximum current efficiency of 7.66 cd/A withoutoptimization of devices. The half peak width of electroluminescence spectra was relativelysmaller than compounds 1a and 1b in Chapter 2 and the color coordinates were (0.34, 0.59).For further enhancement electron injection/transport properties, we designed andsynthesized a yellow-green light-emitting compound in which diphosphine oxide groupsubstituted at thiophene as terminal group at both sides of the core 2,1,3-benzothiadiazole.The electron-deficient functional diphosphine oxide groups may endow the material withgood electron injection and transport properties and high photoluminescence efficiency. Apreliminary exploration of the synthesis, characterization and photophysical properties of thenew compound were done. The device characterizations are currently underway in ourlaboratory.